Method of gas treatment, including impurity removing steps



July 10, 1956 G. H. PALMER ET AL 2,753,701

INCLUDING IMPURITY REMOVING STEPS METHOD OF GAS TREATMENT,

Original Filed July 28. 1949 3 Sheets-Sheet 2 NAW MN Sw Nm.

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July l0, 1956 G. H. PALMER ET AL METHOD OF GAS TREATMENT, INCLUDING IMPURITY REMOVING STEPS Original Filed July 28, 1949 5 Sheets-Sheet 3 ypassageways.

United States Patent O METHOD OF GAS TREATMENT, INCLUDING IIVIPURITY REMOVING STEPS George H. Palmer, Fanwood, N. J., and George T. 'Skaperdas, Flushing, N. Y., assignors to The M. W. Kellogg Company, Jersey City, N. J., a corporation of Delaware Continuation of application Serial No. 107,338, July 28, 1949. This application October 30, 1953, Serial No. 389,195

Claims. (Cl. 62-175.5)

This invention relates to improvements in the treatment of a gas or gaseous mixture to effect liquefaction and/ or fractionation thereof, or to adjust the characteristics of the gas as to composition, temperature or pressure. More particularly, the invention relates to improvements in such treatments as applied to air.

This application is a continuation of application Serial No. 107,338, filed July 28, 1949 now abandoned, on behalf of the same inventors.

Gases, such as air, may be treated to effect liquefaction or fractionation thereof by compressing the gas, precooling compressed gas by heat interchange with cold product, expanding a portion of the cooled compressed gas to perform external work, and passing expanded gas through one or more heat interchange steps, particularly the above-mentioned precooling step, with or without intermediate fractionation of the gas in an expanded condition. The precooling of compressed air may be performed in reversing countercurrent heat exchangers or reversing countercurrent regenerators, such operations hereinafter being referred to generally as heat interchange.

In the use of the reversing heat exchangers at least two passageways are provided in which the air and a product stream are passed in countercurrent heat exchange through the walls separating the passageways, the air and product streams being passed through the passageways alternately in time cycles. When the air is separated into nitrogenrich and oxygen-rich fractions two such reversing heat exchangers may be used, each provided with a pair of Separate streams of the air flow through, each heat exchanger in reversing countercurrent heat exchange with the separate product streams. However, if it is desired to obtain one of the product streams, for example, the oxygen-rich stream, uncontaminated by the impurities which are normally acquired by the product stream in flowing through a reversing heat exchanger in this manner, the air to be treated may be passed in reversing heat exchange only with one of the product streams, for example, the nitrogen-rich stream. In that method of operation a single reversing heat exchanger may be employed, but provided with three passageways. The air and nitrogen streams are passed in reversing heat exchange through two of the passageways while the oxygen stream is passed continuously through the third passageway in counter current heat exchange with the air stream.

Reversing countercurrent regenerators are employed in pairs through which an air stream and a product stream are passed countercurrently, the flow of the streams being alternated between the regenerators in time cycles. The air is cooled by passage through a regenerator which has been cooled previously by the passage therethrough of the product stream, the product stream meanwhile being passed through the other regenerator of the pair. When the air is fractionated into nitrogen-rich and oxygen-rich fractions two such pairs of regenerators may be employed through which portions of the air are sep- 2,753,70i Patented July 10, 1956 'ice arately cooled by the cooling effect of the oxygen-rich and nitrogen-rich product streams. However, when it is desired to recover one product fraction without contamination the regenerators are employed only in connection with the other product stream. The product stream desired in an uncontaminated condition, for example, the oxygen fraction, is utilized to cool a portion of the air in a heat exchanger. The regenerators may have separate passages provided therein through which the oxygen stream may be passed in heat exchange without reversal. The oxygen stream may be passed through such separate passage simultaneously with the passage of the air stream through the reversing passageway of the regenerator. In this operation the air is cooled regeneratively by the nitrogen stream and by heat exchange with the oxygen stream. Alternatively, the oxygen stream may be passed through such separate passageway simultaneously with the passage of the nitrogen stream through the reversing passageway. In this operation the air is cooled regeneratively by both cooling streams. As a further alternative, oxygen may be passed continuously through both regenerators.

When the air is treated to produce a liquid air product, the air feed stream is cooled in the reversing regenerator, or reversing heat exchanger, by the stream of expanded air. Furthermore, when a fraction of the air, such as the oxygen-rich product, is to be recovered in the liquid condition the feed air is cooled only by such products as are in the gaseous condition.

The heat exchangers and regenerators are employed in the reversing arrangements described above in order to clear the equipment of materials deposited from the gas being cooled. Precooling the air brings about the deposition of liquid Water, water ice, and solid carbon dioxide. These deposits accumulate on the heat exchange surfaces of the heat exchangers and the regenerators and must be removed periodically to prevent blockage of the passageways for the air through the equipment. The time cycles for alternating the ow of air and the cooling stream in the reversing heat exchangers and reversing regenerators are arranged to stop the flow of air prior to the accumulation of deposits, in amounts eective to obstruct the air passageways. The air stream is then diverted to the other passageway of the exchanger, or the other regenerator of the pair in use, and the cooling stream which formerly flowed through the other passageway, or other regenerator, is passed through the passageway or regenerator containing the accumulation of deposits to reevaporate such deposits in the cooling stream and thus remove them from the apparatus. In the fractionation treatment of air wherein the air feed is passed in reversing heat exchange relation with a nitrogen-rich product fraction the nitrogen fraction is utilized both as a cooling medium and as a scavenging medium, in that the passage of the nitrogen-rich fraction in reversing heat interchange with the air feed serves both to cool the air and clear the equipment of carbon dioxide and water deposited by the air being cooled. Meanwhile the air feed is cooled and caused to deposit additional material on the metal surfaces of the passageway previously traversed by the nitrogen-rich fraction. It is usually preferred to employ as the scavenging medium a product stream whose recovery in the pure state is not necessary or a product stream from which the material scavenged from the apparatus can be readily removed.

The direction of flow in either heat exchanger passageway or regenerator is reversed as a result of the interchange of passageways between the reversing streams but each of the gaseous streams undergoing reversal always flows through the heat interchange zone in the same direaction, rst in one passageway or regenerator and then in the other. Another stream, or a plurality of streams,

of the product fractions may be caused to flow through other passageways, in the same heat interchange zone, which may or may not be reversed.

Under certain conditions the capacity of the scavenging gas for removing precipitate from the heat exchanger or regenerator is insuiiicient to remove suchdeposit at the same'rate per cycle at which it is precipitated. Under such conditions` the amount of precipitated material in the heat exchange zone accumulates from cycle to -cycle whereby periodic and undesirable shutdowns of the heat exchanger'or regenerator are necessary to effect removal of theaccumulated'deposits. This condition occurs ordinarily in operations in which the difference. in temperature between the warm stream and the4 coldstream, or streams, at corresponding points in the heat exchange zone increased toward the cold end'of the zone. This divergence' of the temperatures ofthe cold and warm streams toward the coldy end of the heat exchange zone occurs'when the product of mass flow rate and specific heat of the warm stream is greater than the product of mass ow' rate and specific. heat of the coldv stream or is greater than the sum of the same products for. allA the streams being warmed when a plurality of cold streams are used. In' other words, this divergence occurswhen the' heat capacity of the warm stream, as deiined bythe product of its mass flow rate and specific heat, is greater than the heat capacity of the cooling uid irrespective of the fact that the heat capacity of the latter may be derived from a single cold stream or from` the. sum of the products of mass flow rate and specific heat of a multiplicity of cold streams when the coolingfiuid'- comprises mo're than one stream. This divergence of the temperatures may be so great as to produce temperature differences between the warm stream and the scavenging stream in the colder portion of the heat interchange zone which are so large as to limit the capacity of the available scavenging' gas for removing the deposits at the rate at which they are formed. In the` treatment of air the carbon dioxide precipitatedk is deposited substantially entirely in the colder portion of the heat interchange zone and in such portion the carbon dioxide is, volumetrically, the principal deposit. Therefore, blockage of the air passage is most likely to occur through insufficient evaporation of carbon dioxide in an area of excessive temperature differentials between the warm and cold streams near the coldV end of the heat interchange zone. Generally, this area is co-extensive with the zone of precipitation of the carbon dioxide.

While the foregoing is the usual application of the invention, it also is applicable to conditions where the heat capacities of the warm uid being cooled and the cooling fluid are such that the temperatures of said fluids converge toward the cold end of the heat interchange zone, butwherein the temperature differences between the iiuids in some parts, usually near the cold end, are still too great to insure complete removal of precipitated deposits in that part. For example., the regenerator or exchanger flow system may be designed for economical or other reasons so as to provide for a great temperature difference at the warm end and to operate with a high average temperature diierence. Under such conditions the temperature difference near the cold end may well be excessive in spite of the fact that the temperatures are converging in that area.

The temperature differences between the warm stream and the scavenging stream which may be tolerated are affected somewhat by the relative quantity of scavenging gas available. When a relatively large volume of scavenging gas is available, relatively great temperature differences between the Warm and cold streams may be tolerated, whereas the necessity for removing` accumulated deposits with a relatively small volume of scavenging gas requires that the temperature difference between the warm and cold streams be substantially restricted. The temperature differential between the warm and cold streams which may be tolerated at any point in an air heat interchange zone, in operations in which all of an impurity which is deposited at that point by the air stream is to be evaporated by the scavenging stream, may be determined for design purposes in accordance with the rule that, for the impurity under consideration, the. saturation capacity ofthe scavenging stream passing that point shall be at least as great as the saturation capacity of the air stream passing that point. As ar factorot` safetyy the ruleI may be modifiedA to require thatv the saturation capacity of the nitrogen stream shall be substantially greater than that of the air stream. In the design of reversing heat exchangers a safe rule is to require that the saturation-v capacity of the air stream shall not exceedthe capacity of the scavenging stream at saturation. A ternperature differential which satises these conditions is permissible for the point under consideration in operationsin which it is desiredfto effect complete-re-evaporation of all of the impurity under consideration which isd'epositediat that point. Separate determinations are re` quired for each impurity being deposited andv reevaporatedandzit is found ordinarily, that in the treatment ofair, the maximum temperature differential permissible when-re-evaporating all deposited-wateris lower tha-nthe maximum permissible temperature differentialr for merely re-evaporating all carbon dioxide.

The temperature diiference may be maintained' sufficiently-srnall' toeffect. complete reevaporation of substantially all of' the precipitated carbon dioxide' and waterioe althoughgenerally-it-maybequite permissible to main'- tain!V the temperature differential abovethe maximum for complete removal of: water ice but below the maximum permissible for removing all of the precipitated carbon dioxide. Operations according to the latter conditions permit accumulation of' residual unevaported water ice between. reversals, but they amount off water ice so accumulated is toosmall to interfere with theV equipment duringthe normal operational on stream periods of the air separation system.

The fractionation or liquefaction of airinvolves a va rietyl ofoperations providing varying quantities of scavenging gas., In the fractionation of air into oxygen-rich and nit-rogemrich product fractions it may be undesirable to. employ the oxygen product fraction as a scavenging stream, whereby only the nitrogen stream is available for this: purpose. Likewise, in operations where a portion of the product is recoveredl in liquefiedv form, the volume of available scavening gas is substantially restricted.

In the fractionation of air to produce oxygen-rich and nitrogerrrich streams which are recovered as gas, both product streams are available for heat exchange with the compressed air, In this operation the massow rate of the'cold' streams is substantially equal to the mass flow rate of: the warmstream. However, atthe lowv temperatures prevailing in the cold endof the heat inter-change equipment the specific heat of the warm stream is subw stantially greater than the specific heat of the expanded cold streams, as a result of the substantial d-iterence in pressure. The difference in speciic heats increases as the temperature falls. In this operation, therefore, the divergence of the temperatures ofl the warm stream and Vthe cold` streams toward the cold end of the exchanger', is affected largely by the efect of the operating conditions on the specic heats of the warm and coldv streams. When it is desired to recover a product stream, such as a portionl of the oxygen-rich fraction, in liquefied form the mass tlow rate of the cold streams is thereby reducedA whereby there is an even greater divergence ofl the temperatures of the` warm. stream and the cold streams towardl the cold end of the heat interchange zone.

In the treatment of air the removal of deposited irnpurity in the heat, interchange zone may bey maintained,

ata rate equalY to the rateof precipitation if the, ratio of the vapor pressure of the impurity at the ternperaturev 0f the scavenging gas, to the vapor pressure. of the,y in;-

avsavoi ,5 purity at the temperature of the air being treated, is approximately equal to, or greater than, the ratio of the actual volume of the air being treated to the actual volume of the scavenging gas. This last-mentioned relationship is subject to some error in view of the migration of carbon dioxide in the heat interchange zone, deviation of the gases from the perfect gas laws, and the diterence in the temperature between the scavenging gas and the air being treated. It is preferred, therefore, that the vapor pressure ratio be greater than the volumetric ratio.

It is an object of this invention to provide an improved method for treating a gas or a mixture of gases in a reversing heat interchange system of the types described above, wherein the cold stream, or streams, pass through the heat interchange zone at a lower weight rate of iiow than the warm stream, or streams, being treated or Wherein the specific heat of the cold stream, or streams, is less than the specific heat of the warm stream, which method provides for restricting the diierence in temperature between the cold stream, or streams, and the warm stream near the cold end of the heat interchange zone to a maximum which permits removal by the scavenging media of precipitated deposits at a rate eiective to keep the reversing passageways open during long operating runs.

It is a further objects of this invention to provide an improved method for treating air in a reversing heat interchange system of the types described above, wherein the air being treated is cooled in said heat exchange system by cold backward-flowing products which flow through the heat exchange zone at a weight rate of flow lower than that of the warm air stream, which method provides for restricting the difference in temperature between the warm and cold streams near the cold end of the heat interchange zone to permit removal of deposits by the scavenging media at a rate effective to keep the reversing passageways open during long operating runs.

It is a further object of the invention to provide an improved method for fractionating air in which the air stream is cooled by backward-owing product stream, or streams, in a reversing heat interchange system of the types described above and wherein the specific heat of such backward-flowing product streams is less than the specific heat of the air stream, and in which method means are provided for restricting the temperature difference between the air stream and the cold product streams near the cold end of the heat interchange Zone to permit sucient removal of precipitated deposits by the product stream, or streams, employed as scavenging media to permit long operating runs.

It is a further object of the invention to provide an improved method for fractionating air in which the air stream is cooled by counterowing product fluid and in which means are provided for restricting temperature differentials between cold and warm streams in a colder portion of the heat interchange zone to permit complete evaporation of carbon dioxide deposits by the cold uid employed as scavenging media.

Other objects and advantages of the invention will become apparent during the course of the following description.

In accordance with this invention these objects are approached by withdrawing from a regenerator or heat exchanger, of the types described above, a portion of a stream of cooling duid passing therethrough and recombining it with the same, or equivalent, stream of cooling fluid about to enter a portion of the heat interchange zone which is colder than the portion of the heat interchange zone traversed by the withdrawn portion of fluid just prior to withdrawal, whereby the portion of cold fluid thus withdrawn is repassed through a portion of the heat interchange zone in which the temperature differential between the warm and cold scavenging stream would be excessive, but for such repassage of cold iluid. Ordinarily, the withdrawn portion of counterfiowing cold fluid is recombined with the same stream of cooling Huid prior to entry of the stream of cooling fluid into the heat interchange zone. In some cases, however, it may be desirable to repass the withdrawn portion through a separate passageway in the heat interchange zone. Ordinarily, the diverted portion cooling iluid is withdrawn from the stream passing through the heat interchange Zone after the stream has passed the warmest point at which there would bean excessive temperature differential but for the withdrawal and repassage of the diverted portion of cooling liuid, although this rule is subject to certain modifications to be described below. In other words, the iverted portion is withdrawn for repassage after the stream of cold fluid has reached a point at which the difference between the temperature of precipitation and evaporation of a deposited component is suciently small in the absence of said withdrawal and repassage of cold tlnid to permit substantially complete evaporation in each operating period of all the components precipitated at that point in the preceding operating period. In the treatment of air it is desirable ordinarily to withdraw a portion of the stream of cooling uid for repassage after it has passed the zone of precipitation of carbon dioxide, although it may be necessary to divert at least a part of the material to be repassed only after it has passed ya substantial distance beyond the region of precipitation of carbon dioxide if it is necessary to insure complete evaporation in each reversal of all water ice. The Withdrawn portion is then recombined with the same or equivalent cooling stream prior to passage thereof through a zone in which temperature differential would otherwise be excessive. Conveniently, and preferably, the withdrawn portion of the cooling stream is recombined with the stream outside the heat exchanger or regenerator, the combined stream then being passed into the heat interchange zone. The eiect of this operation is to cool the warm stream, at some points in heat interchange with the repassed cooling fluid, to lower temperatures than would be reached in the absence of this operation. Thus the warm stream may be said to be further cooled, even though the exit temperature of the warm stream, in the absence of other adjustment of the operation or change in apparatus design, may be higher than that reached in the operation in the absence of the present invention. The temperature of the cooling stream, or the average temperature of the cooling streams, also may be lowered, but to a lesser extent, in -a portion of the zone between the points of diversion and recombination. It is understood that temperature of the cooling stream, or the average temperature of the cooling streams, means the temperature of a single stream having the same cooling effect as the multiple streams at any one point in the heat interchange zone. However, the effect on the Warm stream and the cold streams in the region traversed by the repassed cooling fluid is to reduce substantially the diiference of the temperature between the warm stream and the cold streams. In this manner the capacity of the scavenging stream or streams for vaporizing a deposited impurity is made effective to remove such impurity at the same rate per cycle at which it is deposited.

A portion of the stream of cooling iluid thus withdrawn may be separated from the stream after it has passed partly through the heat interchange zone or after it has passed completely through the Zone. The cooling stream from which a portion is thus diverted may be a stream which passes in reversing heat interchange with the warm stream or it may be diverted from a cooling fluid passing continuously in non-reversing heat interchange with the warm stream. Advantageously the ternperature conditions in the heat interchange zone may be adjusted in accordance with this invention by simultaneously and separately withdrawing and recirculating portions of both the separate streams of cooling fluid owing through reversing and non-reversing passages of the heat interchange zone.

Since any cold stream introduced into the heat interchange zone at the cold end thereof is continuously warmed as it owsthrough the heat interchange zone, theportion ofthe coolingl fluid which is diverted, in accordance withthisinvention is `Withdrawn from the heat interchange zonev afterit has been warmed substantially above its entering temperature. Preferably the portion to be divertedy is withdrawn only after it has passed through the heatinterchange Zone a distance sufficient to warm it above the temperature of substantial precipitation of a deposited impurity, such as carbon dioxide, whichmight otherwise accumulate excessively.

' The portion of cooling fluid is withdrawn from the heat interchange zone and, if convenient, is recombined with other, and colder, cooling iluid. Preferably the withdrawn portion is so recombined with cooling fluid by introducing it into the same stream of cooling fluid from which it is withdrawn,'but at a colder point. it there are a plurality of streams of similar composition flowing through the heat interchange zone as cooling media vthe withdrawn portion of cool fluid may be recombined with the same or a similar stream. If one of the streams of cooling fluid isto be discarded after passage through the heat interchange zone a portion of cool uidwithdrawn from a stream of different composition maybe repassedA through at least a portion of the heat interchange zone, in accordance with this invention, by combining it with. such stream of cooling uid to be discarded. The withdrawn portion of cooling uid may be repassed through the heat interchange zone through a separate. passageway therein.

The portion of cooling fluid withdrawn and recombin'ed` with cooling fluid for repassage in heat exchange with the zone of precipitation, in accordance with this invention, may be so recombined with cooling uid without anyY treatment to adjust its temperature or composition. However, if the volume of cooling fluid withdrawn from the heat interchange zone for repassage is large in relation tothe volume of the stream of cooling fluid with which it is` to be recombined, the temperature of the resulting combined stream may be made to differ substantially from the temperature of other streams of cooling fluid flowing through` the heat interchange zone. Under such. conditions it is desirable to subject the withdrawn portion of cooling uid to heat exchange to cool it, prior to recombining it with other cooling iiui'd or prior to passing the combined stream into the heat interchange zone. Such cooling of the nwithdrawn portion may be desired also in. order to lower the temperature of the cooled gas stream emerging from the cold end of the heat interchange zone. The diverted portion of cooling fluid may be cooled. by heat exchange with any suitable stream of'colder uid, or the diverted portion may be recombined with other cooling uid and the combined stream may then be'treated. to adjust its temperature prior to entering the heat interchange zone. For such temperature adjustment the diverted portion, or the combined stream, may be passed in heat exchange with any cold stream emerging from, or about to enter, the cold end of the heat interchange zone.

If the portion of cooling fluid withdrawn from the heat interchange zone for repassage is obtained from a scavenging stream the portion soV withdrawn will contain evaporated impurities. For example, in the fractionation of air in which a portion of a nitrogen-rich scavenging stream is withdrawn for repassage, the carbon dioxide and water contained in such withdrawn stream may be prt-:cipitatedv upon recombining any withdrawn stream with colder cooling huid. Consequently it may be dcsirableto provide some means for removing the impurity contained in a portion of a scavenging stream which is withdrawn for repassage. This may be done by any suitable treatment before or after recombination of the withdrawn kportion of cooling uid with other colder iiuid. Conveniently, the combined stream may be filtered prior to any Yfurther heat exchangev thereof to remove precipitated impurities.

The amount `of cooling uid withdrawn and repassed through at least a portion of the heat interchange zone',V inV accordance with the principles discussed above, should be limited to the minimum necessary to produce the result'fdesired in the'complete removal of deposited impui-ities by the, scavenging fluid as it is desirable, for reasons of economy, to avoid unnecessarily low differences between'the temperatures of the cold and warm streams at the cold' end of the heat interchange zone and any unnecessary increase in the'di'fference in the ternperatures between the'cold and warm streams at the warm end of the heat interchangey zone. In general, the practice of the invention requires diverting and repassing cooling uid quantities such that in the portion of the heat interchange zone traversed by the divertedv fluid the total mass flow rate of cool fluid exceeds the mass` ow rate of warm fluid by a suiicient margin.

An important application of the invention relates to the fractionation of air to produce oxygen-rich and nitrogenrich streams. In such a system the cooledcompressed air is passed to a fractionating system comprising a fractionating zone,` conduit lines for conductingY the cooled compressed air to the fractionating Zone, and conduit lines for conducting one or more product fractions from the fractionating zone to the heat interchange zone. In suchV a fractionation treatment of air the Withdrawn portion of cooling uidV may be obtained' from either, or both, o f the product streams when these products are obtained in the gaseous phase. The fractionationY treatment of air may be arranged to produce a relatively pure liquid oxygen product,l in which case the other fraction, consisting of relatively impure nitrogen,v

is passed through the heat interchange system to supply substantially all of the cooling and scavenging treatment and the withdrawn portion is taken from this fraction.y The improved method ofthis invention may be applied to all of these various arrangements, or other arrangements in air liquefaction, in accordance with the general principles described above.

Further explanation of the present invention will be made with reference to the accompanying drawings, which illustrate the application of the invention yto air fractionation. It will be understood that the principles of the invention are also applicable to the treatment of any gasto effect fractionation or liquefaction thereof. It is to be understoodalso that reference to the drawings is by way of example only and is not restricted to the physical limitations of the apparatus indicated in the figures, wherein: l

Figure 1 is a diagrammatic representation of an arrangement of apparatus adapted to illustrate the invention in a process for treating air by liquefaction and fractionation to separate it into gaseous oxygen-rich and nitrogenrich product fractions. In this representation the ternperature adjustment is obtained by diverting from the heat interchange zone partof the nitrogen-rich product fraction, after it has passed a sufficient distance through a reversing 'heat'exchangen and recombining the diverted portion with the same stream as it is about'to flow into'the heat exchanger.

Figure 2 is a diagrammatic representation of apparatus employed in a modification of the heat exchange arrangement of Figure 1.

Figurel 3` is a diagrammatic representation of apparatus employed in afurther modification of the process, as applied to systems employing reversing heat exchangers.

Figure 4 is a diagrammatic representation of apparatusillustrating the method of the invention applied to a process arrangement employing reversing regenerators in an air treating system.

In the processV arrangement illustrated in Figure 1, air is broughtv intothe processing system through line 1 to an air lter 2' for removal of dust or other suspended matter. The filtered air then passes through line 3 to compressor 4 which is providedwith two stages of compres- 9 sion. An inter-cooler is provided in connection with compressor 4 and the air flows from the first stage of compression through cooler 5 prior to its passage to its second stage of compression. In compressor 4 the filtered air is compressed to a pressure of approximately 100 pounds per square inch absolute. This pressure and the other condition of temperature given in the following description of the process exemplify suitable conditions of operation. The compressed air flows from? compressor 4 through line 6 into an after-cooler 7 wherein the compressed air, which emerges from compressor 4 at a temperature of about 300 F., is cooled to remove the heat of compression. Such cooling also condenses the greater part of the Water vapor contained in the compressed air.

The mixture of air and water condensate flows from after-cooler 7 into separator 8 from which the condensate is withdrawn through drain line 9. The air passes from separator 8 through lter 10 which is provided to remove any suspended or entrained matter which might otherwise be carried through latter stages of the process. The filtered compressed air ows from filter 10 through line 11 to reversing heat exchanger 12 at a temperature of about 90 F. and at a pressure of 99 pounds per square inch absolute.

Reversing heat exchanger 12 is provided to effect countercurrent heat exchange of the compressed air with cold nitrogen-rich and oxygen-rich product streams. ln heat exchanger 12 the compressed air is cooled by these streams of cooling fluid to a low temperature which may be near the temperature of liquefaction, for example, approximately 253 F.

Heat exchanger 12 is shown diagrammatically in Figure l as being provided with three passageways; 13, 14 and 15. These passageways are so constructed to provide for a most eflicient thermal contact between them for heat exchange throughout their length. The flow of compressed air and nitrogen product is alternated between passageways 13 and 14 of exchanger 12 in suitable periods of, for example, about three minutes in order to employ the nitrogen stream for scavenging from the heat exchanger the water and carbon dioxide precipitated from the air stream. For this purpose line 11 is connected to a reversing valve 18 which connects with lines 16 and 17 which in turn connect with the ends of passageways 13 and 14, at the warm end of exchanger 12. Lines 16 and 17 also connect with a reversing valve 19, similar to valve 18. Valve 19 also connects with a discharge line 20 through which the nitrogen-rich product is withdrawn from the process, at a temperature of about 82 F. Valves 18 and 19 are operated periodically, by an automatic timing device not shown in the drawing, whereby valve 18 directs the compressed air from line 11 into line 16 at the same time that valve 19 is operated to exclude compressed air from line 17 and permit the flow of the backward returning nitrogen product stream into line 20, and vice Versa.

The cold end of passageway 13 connects with line 25 which in turn connects through check valves 22 and 24 with line 28 and 26 respectively. Similarly, the cold end of passageway 14 connects with line 27 which in turn connects through check-valves 21 and 23 with lines 28 and 26 respectively. Line 28 is the source of the nitrogen-rich product stream which flows to reversing heat exchanger 12 from the fractionating system in a manner to be described below. Line 26 is provided to transmit to the fractionating system the cold compressed air emerging from the cold end of the reversing heat exchanger 12. Check-valves 21 and 22 are arranged to be held closed by air pressure from line 27 or line 25 while check-valves 23 and 24 are arranged to be held open by such pressures. The nitrogen pressure from line 28 opens whichever of valves 21 or 22 is not subjected to air pressure.

The cold compressed air from heat exchanger 12 llows into line 26 in the manner described and through line 26 into a surge drum 53 which is provided to separate from l0 the air any solid carbon dioxide which may be present, particularly during starting-up periods of operation before exchanger 12 has been chilled to normal operating temperature.

The air leaves Surge drum 53 through line 54 which connects with lines 55 and 69. The air is distributed from line 54 into lines S5 and 69 in proportions which` depend upon the capacity of the plant. As an example, for a plant of moderate capacity, 60% of the air may be taken through line 55 which connects to a liquefier vessel 56 wherein the air is passed in heat exchange with cold backward-returning nitrogen-rich product. By the indirect heat exchange with cold nitrogen in the liquefier 56 the air is cooled suficiently to effect partial liquefaction thereof. The resulting mixture of liqueed and gaseous air then ows from liquefier 56 through line 57 at a temperature which may be, for example, 274 F. The mixture of liquefied and gaseous air flows through line 57, containing pressure control valve 58, into the inside of a reboiler heat exchanger 59 which is located in the base of a fractionating column 60 and arranged to be submerged in a pool 61 of liquid oxygen at the bottom of the interior of fractionating column 60. The process is arranged to pass the air to reboiler 59 at a temperature somewhat higher than that of the liquid oxygen in pool 61 so that the resulting heat exchange reboils the liquid oxygen as required by the fractionation treatment and further cools the air, for example, to about 278 F. whereby condensation of the air is substantially completed in reboiler 59.

Liquefed air is withdrawn continuously from the bottom of reboiler 59 through line 62. A drain line 63 is provided to permit separate draining of reboiler 59. Line 62 connects with a filter 64 to permit passing the liquefied air through a body of granular filtering material to remove any solid particles, such as carbon dioxide or hydrocarbons such as acetylene. The filtered liquefied air passes from filter 64 through line 65 to a sub-cooler 66 in which it is further cooled by heat exchange with cold nitrogen-rich product from the top of fractionator 60. The cooled liquid air ilows from sub-cooler 66 through line 67 which is provided with a pressure reduction valve 68 and connects with the upper portion of tower 60. The air is expanded on passing through valve 68 to a pressure of about 25 pounds per square inch absolute, and the sub-cooling is provided at 66 to reduce the temperature of the liquid air to a level at which it does not vaporize extensively when the pressure is reduced at valve 68.

The remainder of the cooled compressed air ilowing from drum 53 through line 54, is passed through line 69. In accordance with the example given, the air flowing into line 69 would amount to about 40% of the cold compressed air emerging from drum 53. The cold compressed air passes through line 69, which is provided with a strainer 69', to the inlet of an expansion enine 70. The air is passed through expander 7i), with the performance of external work, to an exit pressure of approximately 25 pounds per square inch absolute. This expansion with performance of external work lowers the temperature of the air to approximately 304 F. Under these conditions of temperature and pressure the expanded air is 1n vapor form and flows from the exit of expander 70 through line 71 to an upper intermediate point of fractionating column 60. A surge and trap-out drum 72 may be interposed in line 71 to minimize pressure surges in this line and remove solid carbon dioxide which may be present during starting-up periods.

In fractionating column 60 the air is fractionated under conditions effective to separate an oxygen-rich product in the lower portion of a column and a nitrogen-rich product at the top of the column. The low temperaturel necessary in the top of the column is provided by the sub` cooled air from line 67 and the expanded air from line 71 and the reboiling necessary in the bottom of the column is provided by reboiler 59 in the manner described. Fractionating tower 60 is provided with gas and liquid contact means' '7-4 which may" be bubble captrays, or similar meansffor assisting 'fractionation of the air into oxygenrichl and nitrogen-rich products.

The oxygen-rich product fraction is Withdrawn from fractinating column 60 throuh line 75 which connectswith'column 60 at a point above but near the pool of liquid oxygen at 61. The oxygen vapors pass through line '75 into passageway 15 ofheat exchanger 60.` P as.- sageway is a non-reversing passageway through which the'oxygen product stream passes continuously in countercurrent heat exchange with incoming air feed iiowing through either of passageways-13 or 1.4. After passing, throughpassageway 15 of the heat exchanger the oxygenproduct'A is dischared from the system throughy line 76.

The nitrogen-rich product fraction is with-drawn over head as a vapor from the top of column 60 through line 77fwhlich connects with sub-cooler 66. The nitrogen product is withdrawn from column 60 at a temperatureofwapproximately -307 F. and passes' throughf subcooler 66 in heat exchange with liqueiiedair passing therethrough to effect sub-cooling of the latter, in the manner described above. From sub-cooler 66 the nitrogen-product passes by way of line 78- into liqueer 56 through which itflows in heat exchange-with the cold' compressedair from heat exchanger 12f In. this opera-V tion the nitrogen vapors are warmed to a temperature of' about 275 F., at which they are discharged from liqueer 56 into line 37 for transmission to-'heat exchanger 12.` Under some conditions of operations the cooling in liqueer 56 requires flowing less than all of thenitrogen product stream from line 78 throuh the liquetier for use under such conditions. Line 79, providedfwith Valve 80, is provided to connect line 78v directly with line 37, bypassing liqueer 56.

The operation described above is that followed during equilibrium conditions after the preliminary adjustments in-theoperation of the apparatus during the starting-up period have been completed. During the starting-up period it may be undesirable to pass expanded-fair from line 71 into tower 60. Line 73 is provided for use-at that time. Line 73 connects 71 directly with line '79 to permit discharging the expanded air from expander 70 directly into line 37 for passing into heat exchanger 12.-

'The nitrogen stream in line 37 passesr to-line 28,. through either of filters 38 and 39, and then into one of passageways 13 and 14 ofl heat exchanger-12. Heatexcha-nger'l is shown as a multi-stream heat exchange apparatus: havingfthree. passageways for the` ow ofi air an'doxygenlrich` and nitrogen-rich pro-ducts.- Passageways 13 and-14. of the exch-anger are reversing passages-and are arranged to carry, alternately, compressed air and nitrogen-rich product streams. Passageways 13 and 14 are similar in ow resistance and heat exchange characteristics. Passageway 15 usually is constructedsimilarly to the other passageways but is designed to carry only the oxygen-rich product in countercurrent heat exchange relation withthe compressed-air and nitrogen. streams.

The relation lof passageways 13, 14 and 15-ofv heat-.exchangerl-Z is indicated diag-rammatically'in` Figurel but itV will be understood that in this type of heat ex'- changerV the heat exchange elements providerapid and efficient exchange of heat between each passageway and eachother passageway. It is desirable for eicient heat exchange to have. all passageways of the heat exchanger packed with a metallic material. This-'packingfmaterial may be ot any suitable character and may consistof a multiplicity of closely spaced pins,l coils, edge-wound metallic ribbon, longitudinally spaced metal elements, or the like. For reasons of thermal eciency it is preferable that the metallic packing be ai-xed to the walls of thel passageways either by furnace brazing or--with-a suitable"met'al'tometalbonding material, such -assolder. When the individual` passageways of the, exchangerfare-` comprised ofV a`plurality of conduits, such conduits are preferably metal bonded into an integral unit and the passageways themselves-also are metal bonded together for,

14 of the exchanger in countercurrent heat exchange re-4 lation with-the cold products of the separation-gives; up enough heat to these products to be substantiallycooled to a row temperature, which may be near air-,liquefactionz As the air temperatureis reduced,- water,-

temperature. first as liquid and then as ice, and solid-carbon dioxide, precipitate from the air and are deposited in theexchanger. The zone of precipitation of the carbonl dioxide is. near the cold end of the exchanger while most-A of. the waterl precipitates nearerthe warm end.. Were the-,air and nitrogen-rich product not interchanged between passage-V ways 13 and 14, accumulation of solids, such as formed by carbon dioxide,v eventually wouldcompletely obstructthe exchanger. However, reversi-n g valve-13, with valve 19- cooperating therewith, periodical-lydirects the airinto theJ alternate passageway' which hasbecn. carrying the nitro@v gen-rich product and this change in tlow causes checkvalves 21, 22, 23 and 24 to respond-to the resulting pressure-changes,I so-that, for example, the air which-has been owingfrom the passageway 13 throughl line 25: and check-valve 24 now leaves passageway i4Y through 27- and check-valve` 23. Meanwhile, the nitrogenrich product thathas bcen'owing: from line- 28 Vthrough check-valve 21- into line 27 and passageway 14,-. is caused to enter passageway 1-3 by wayV of check-valvez 22: andi line 25. In owing through the passageway previously. traversed by the airstream'the nitrogenremoves'precipitate from the air, the water and carbon'dioxidetbeingavaporized intozthernitrogen stream:

In this manner, the'nitrogen-rich product is utilized not only to abstract heat from the inii'ow-ing compressed' air but also to' evaporate and remove higher boiling components that have been deposited' from the air inA the'reversing passageways. as zu result of the'- temperature reduction. The nitrogen-rich stream ist.. therefore; a scavenging stream aswell as a cooling.mediturluy Inasmuchas this latter stream is .comprised oi-af product obtained from the separation `of air aftenpressure-reduo tion; itI has greater4 capacity to: holdr water or 4carbon dioxide in the vapor state thandoes the. compressed-air stream atthe. same"Y temperature. This' makes-"possible substantial evaporation-ot deposits inl' spite ofthesma-ller quantity of material in the scavenging str-earnV relative to the quantityoficompressed. air. However; in-the operation illustrated by Figure l, the*4 nitrogen-andioxygenarich streams howto' heat exchanger 1hr-2 from the fractiona'to'r 6i?v a-t a mass ow rate which'v isfat'- most equal= tof-that of the air'strearn. reason that the specic heat. of-thesel product streams is-lo-wer than that ofthe Yair thereoccursza .substantial divergence between theV temperatures of' theproduct streams and the Vair stream toward'y thev cold end'-, of iex'u changer 12... in operations'exemplied :by the? process= of Figure l, the temperature differences ,afi-ei lto'tmzgreatto permit complete evaporation of deposits, by theV nitrogen scavenging streamregardless of its-lowerpressuref- This excessive. differential `between the temperature of .precipi tation andree-vapora-tion:v impedes pri ncipra'liy.- the*- removal of the carbon-,dioxide as precipitation ,otthe; Water-.is

For this` reasonE andi forthe furtherproduct toward the cold end of the heat exchange zone are reduced by diverting and repassing a portion of the nitrogen-rich product from the exchanger after it has passed, partly or entirely, through the heat exchanger. Preferably the diverted portion is removed at a point at which the nitrogen has passed through the heat exchanger a distance suicient to heat it to a temperature in the range of 80 F. to 185 F., for example, a temperature above the temperature of sublimation of the carbon dioxide. Although diversions' are usually based on conditions that take cognizance of the fact that water ice precipitated in the region of carbon dioxide precipitation is for all practical purposes negligibly small and for this reason any blockage by deposits of solid is chiey caused by carbon dioxide and that the presence of water ice may be disregarded, the invention includes within its scope effecting diversions under conditions suitable to ensure the removal of both water ice and solid carbon dioxide from such region. For this purpose a conduit line 29 is connected to passageway 14 for withdrawing a portion of the nitrogen-rich stream when it ows therethrough, the point of connection between line 29 and passageway 14 being determined by the temperature at which it is desired to divert a portion of nitrogen-rich stream. A similar line 30 connects with passageway 13 in the same manner and for the same purpose.

The diverted portion of the nitrogen stream is' withdrawn through line 29, or line 30, depending upon whether the nitrogen-rich product is flowing through passageway 14 or 13 respectively. From one or the other of these lines the withdrawn portion passes through a reversing valve 31 and line 32 into a surge drum 33, which serves as a reservoir for a blower 35. To remove condensate from drum 33, valved-drain line 34 is provided, although normally no condensation occurs. Blower 35 introduces the relatively warm diverted portion of the nitrogen-rich product, by way of line 36, into line 37 through which the main stream of nitrogen-rich product flows as described. Thus, the diverted portion of the nitrogen stream is recombined with the same stream from which it was initially withdrawn, the temperature of the combined stream being about 262 F. The proportion of the nitrogen stream diverted through line 29, or line 30, corresponds to the quantity necessary to achieve the desired result, the exact quantity of diverted gas being aiected by the temperature of the diverted portion. Since the nitrogen-rich product is scavenging the passageways 14 and 13 of carbon dioxide and Water, these substances are contained in the portion diverted through line 29 or line 30. Under certain conditions of operation, for example, when the temperature of the withdrawn portion is warmer than 80 or colder than 185 F., these constituents of the withdrawn portion are precipitated at the temperature of the combined stream. If the quantity precipitated is large it is necessary to provide means to remove such solid particles from the commingled stream, and filters 38 and 39 are provided for this purpose. Filters 38 and 39 contain, preferably, iilter cloth membranes or beds of granular filtering material for removal of ice or solid carbon dioxide. A pair of these filters is provided to permit periodic regeneration. The out-lines 44 and 47 of lters 38 and 39 connect with line 28 for passing the filtered nitrogen stream through heat exchanger l2 as described above. Valves 41, 42, 45 and 46 are provided in lines 40, 43, 44 and 47 respectively, to permit passing the nitrogen to be iiltered through either iilter. Regeneration may be eiected by means of air from which carbon dioxide has been substantially removed. For example, air may be diverted from line 26 through line 4S, heated by means not shown, and passed also by means not shown into lines 44 or 47 through lines 49 or 51 for passage through the filter being revivied. Lines 50 and 52 are provided to withdraw the spent revivifying air from line 40 or line 43.

By this operation a controlled portion of the nitrogen stream being passed through the heat exchanger is withdrawn and repassed through at least a portion of the reversing passageway, combine-tl with the main stream of the same product. Ordinarily, the diverted portion of the nitrogen stream is withdrawn after it has passed sufiiciently through the heat exchanger to be warmed to a temperature above the temperature of sublimation of the carbon dioxide. By withdrawing a portion of the nitrogen at that point, and recombining it with the nitrogen stream about to enter the heat exchanger, the mass ow rate of the cooling fluid passing in heat interchange with the zone of precipitation of the carbon dioxide is substantially increased. By this means the divergence of the temperatures of the cold and warm streams toward the cold end of the heat exchanger can be halted or restricted in portions ot the heat exchanger adjacent the zone of precipitation of the carbon dioxide, whereby the difference between the temperatures at which the carbon dioxide is precipitated and reevaporated is maintained suiiiciently small in all points in the zone of precipitation to permit complete evaporation of deposited carbon dioxide in each passage of the nitrogen stream through the zone of precipitation. The effect of withdrawing and repassing a portion of nitrogen in this manner may be a mere reduction in the rate at which the temperatures of the cold and warm streams diverge toward the cold end. However, the amount needed to be thus diverted and repassed in most operations results in convergence of the temperatures of the cold and warm streams toward the cold end in that portion of the heat exchanger between the point of diversion and the cold end.

In the example of Figure l the step of diverting nitrogen from heat exchanger 12 through line 29 or 30 and recombining it with the nitrogen stream in line 37 has the preliminary effect of warming the nitrogen stream entering heat exchanger 12 as a scavenging medium. Consequently the temperature of reevaporation in at least a portion of the zone of precipitation of carbon dioxide may be higher than it would have been but for the diversion and repassage of a portion of the nitrogen stream. At the same time the increase in the volume of the nitrogen stream has the eiect of cooling the air stream to a lower temperature, in at least a part of the heat exchange zone which lies between the point of diversion and the cold end of the exchanger, and particularly in that part of the zone of precipitation adjacent the point Iof diversion and on the cold side thereof. The net eiect is to bring closer together the temperatures of precipitation and the temperatures of reevaporation of the carbon dioxide. The increase in the mass ilow rate of the scavenging stream, and the intermediate removal of solids in filters 38 and 39, substantially increases the capacity of the scavenging stream for evaporating deposits, though the most important elect is that due to the decrease in the temperature diierential between deposition and evaporation of the carbon dioxide.

The exact point at which a portion of the nitrogen stream is to be withdrawn from the exchanger for repassage should be located, for best operation, on the warm side of the area in which substantial precipitation of the carbon dioxide from the air first occurs during passage of' the air stream through the exchanger. The location of the point of diversion of nitrogen is affected by the presence o'r absence of means to separate carbon dioxide and water vapor contained in the diverted stream. If the stream is diverted from a relatively cold point in the heat exchanger it may be introduced directly into the main stream of cold nitrogen and the combined stream may be then subjected to ltration to separate any solid carbon dioxide and water ice precipitated when the diverted nitrogen stream is mixed with the much colder main stream of nitrogen. If the nitrogen is diverted from a warm portion of the heat exchanger it may be desirable to subject the stream of diverted nitrogen to caustic treatment and drying treatment to remove carbon dioxide and water vapor prior to commingling the diverted stream with the main stream of nitrogen.

If a lter, or other means, is used to remove carbon dioxide and water contained in the diverted nitrogen stream, the location of the exact point of diversion is based on a consideration of two factors. The lir'st factor is that the greater the temperature rise of the diverted portion, i` e., the greater the distance travelled by it in the heat exchanger, the smaller is the quantity needed to be repassed. The second factor is that the smaller the temperature rise, i. e., the less the length of the heat exchanger travelled, beyond the minimum necessary to carry it past the area of substantial precipitation of carbon dioxide, the greater is the average temperature difference between the cold and warm streams and the greater the economy of heat exchange.

The location of the point of diversion of a portion of the nitrogen stream, or the amount of the nitrogen stream to be diverted, having been selected, the amount of nitrogen to be diverted, or the location of the point of diversion, may be arrived at by a method illustrated in the following operation. In this operation the calculation is based on the removal of carbon dioxide, it being understood' that a similar calculation which is based on the water content of the inowing air stream at various points in the heat interchange zone will be necessary in determining the amount or the temperature of the cooling fluid be diverted for repassage when it is desired to provide for the complete evaporation of the ywater ice from the same general region.

First the difference in temperature permissable at the coldest point in the zone of precipitation of carbon dioxide in the heat exchanger is determined. For this purpose the quantity of carbon dioxide in the air stream at its coldest temperature in the zone of precipitation of carbon dioxide, that is, at the exit of the cold end ofthe heat exchanger, is calculated. Then the temperature of the scavenging nitrogen stream at that point at which the nitrogen will carry as a vapor a quantity of carbon dioxide equal to the quantity held' by the air stream is calculated. As a factor of safety an amount less than 100% of the saturation value for the nitrogen stream may be taken. The difference between the temperature assumedfor the air stream at that point and the temperature calculatedV for the nitrogen stream may be taken as the allowable temperature differential to be maintained in the Zone ofvprecipitation at the selected point.

Next the quantity of heat which must be absorbed by the nitrogen to be repassed is determined. For this purpose the heat balance for the heat exchanger between the warm end and the point selected for calculation in the preceding paragraph, thatlis, the cold end of the exchanger is calculated to determine a theoretical air exit temperature. This calculation is based on the known temperature of the air stream entering the heat exchanger, the temperature desired for the nitrogen stream leaving the exchanger, the nitrogen temperature at the cold end, calculated in the preceding paragraph, the temperature desired for the oxygen stream leaving the exchanger, the expected temperature of the oxygen stream at the cold endvof the heat exchange zone involved in the calculation, assuming nowithdrawal ofcold uid. The difference in the heat content of the air stream at this calculated theoretical air exit temperature and atl the exit temperature assumed for the air stream in the preceding paragraph is the heat which is to be absorbed by the nitrogen to be repassed in the heat exchanger. The selected quantity of nitrogen to be diverted must ow through-the exchanger far enough to absorb this amount of heat, ornitrogen at a temperature selected for diversion mustbe withdrawn for repassage in an amount which has absorbed this quantity of heat.

In the modification of the process illustrated in Figure l tilters 38 and 39 are provided to illustrate means( for removing the carbon dioxide and Water carried `by the diverted nitrogen stream. However, the process is not limited to applications including means for separating such water and carbon dioxide. If the point of diversion of nitrogen from the heat exchanger is carefully selected the carbon dioxide need not be precipitated upon admixture of the diverted portion with the main stream of nitrogen. The amount' of water vapor carried by the diverted nitrogen is sufciently great to` cause precipitation upon `admixture of the diverted nitrogen with the main stream. However, if the diverted nitrogen stream is withdrawn from a cold point in the heat exchanger the quantity of water precipitated is quite small and does not interfere with the operation of the process during the usual operating runs. If necessary, it is possible to separate it by simpleV means such as a baffled surge drum or a filtering device which operates for long periods of time without cleaning.

As pointed out above, the colder the diverted nitrogen` stream the greater the quantity thereof which must be repassed. Consequently the nearer the cold end of the nitrogen stream the diverted portion is taken, and the greater the quantity necessary to be diverted, the warmer must be the combined stream to maintain the carbon dioxide content of the diverted stream in vapor form in the combined stream. The extent to which the temperature of the main stream' of nitrogen may be adjusted'to higher temperatures, to maintain the temperature of the combined stream at a satisfactory level, is limited by the exit temperaturerequired for the air stream. If the latter is too high the quantity of carbon dioxide retained by the air stream and deposited in the apparatus in later stages of the process becomes excessive for economical opera'- tion. `Ordinarily these considerations limit the lowest temperature of the diverted nitrogen stream to about 185 F. in order to maintain the air exit temperature at a desirable low limit. To provide still better removal of suticient carbon. dioxide from the air stream in the exchanger the temperature at which the nitrogen is diverted should be no lower than about 120 F. and for best operation should be not lower than about 100 F.

The upper limit on the temperature at which the nitro'- gen stream may be diverted, in operations not employing a rlter, or equivalent means for separating the carbon dioxide and water from the diverted nitrogen stream', is controlled by the rapid increase' in the water content of the nitrogen stream at temperatures above about F. Consequently, for operations not employing a lter' or equivalent means, the nitrogen should be diverted from the heat exchanger at a temperature in therange of 80 to 185'F. preferably 8'to 120 F., and Still more preferably 8,0 to F.

This relatively narrow range of preferred temperatures for withdrawal of the diverted nitrogen stream extends through only a small portion of the length of the heat exchanger. It may be desirable therefore to provide a multiplicity of connections extending over a substantial length vof the kheat. exchanger to permit changing the point of diversion of the nitrogen stream in response to changes in operating conditions. l

The yforegoing discussionv of the factors involved in properlyy locating the point of diversion of a portion of the nitrogen stream passing through the heat exchanger relates to an operation `in which nitrogen is withdrawn at only one'point for repassage, that point preferably being located between vthe zone of maximum precipitation of the .carbon dioxide and the warm end of the heat exchanger. However, the invention includes within its scope'operations in which nitrogen, or other cold Huid tov be repassed through the heat exchanger is withdrawn simultaneously from ya'. plurality of points along the length ot the exchanger. The invention also lincludes within its scope operations inwhich latleast one of a plurality of points of simultaneous withdrawal of nitrogen is located within the zone of precipitation of carbon dioxide in the heat exchanger. Such an operation may be advantageous as it permits restricting to the minimum the quantity of 17 nitrogen withdrawn for repassage and permits maintaining a high average temperature differential, thus promoting eiciency.

There is set forth above a description of a method of calculating the quantity or temperature of the nitrogen to be diverted from the nitrogen stream at a single point located in the heat exchanger between the zone of precipitation of carbon dioxide and the warm end of the eX- changer. In the calculation described, the cold end of the exchanger is taken as the point for determining the allowable temperature differential for reevaporating the carbon dioxide. This is satisfactory when withdrawing the nitrogen only from a point or points located between the zone of precipitation of carbon dioxide and the Warm end of the heat exchanger, as it is found that if sufficient nitrogen is thus repassed to provide the allowable temperature differential at the cold end the temperature differentials in the remainder of the heat exchanger traversed by the repassed nitrogen will be below the allowable maximums. However, the quantity of nitrogen which must be repassed through any portion of the heat exchanger, to maintain the temperature diierential under the maximum permissible, increases in the direction of the cold end of the heat exchanger. This is the accumulative effect of two factors. One factor is the increase in the heat capacity of the air as it becomes colder. This results in a normal increase in the temperature differential between the cold and warm streams as the cold end of the heat exchanger is approached. The other factor is that the maximum temperature differential which is permissible at any point in the zone of precipitation to maintain complete evaporation of deposits decreases in the direction of the cold end of the heat exchanger. Consequently, the quantity of the nitrogen stream which must be repassed to maintain satisfactory temperature differentials progressively increases in the direction of the cold end of the heat exchanger.

Therefore, the withdrawal of nitrogen, to be repassed, only from a point or points located between the Zone of precipitation and the warm end of the heat exchanger may not provide maximum efliciency of heat transfer, as the temperature differential thus maintained in the zone of precipitation may at some points therein be substantially lower than the maximum allowable temperature differential. In order to carry out' the heat exchange under economical conditions, it may be desirable to locate the warmest point of withdrawal of nitrogen as near the cold end of the heat exchanger as is possible for carrying out the invention, and divert other portions of the nitrogen stream from points intermediate the warmest point of withdrawal and the cold end of the heat exchanger. For such operation additional draw-o lines 291 and 292 are provided in connection with passageway 14 of heat exchanger 12 and additional draw-off lines 301 and 302 are provided in connection with passageway 13 of heat exchanger 12. These additional draw-off lines connect with the heat exchanger at points intermediate lines 29 and 30 and the cold end of the exchanger. Line 291 connects with line 29 and line 292 connects with line 291, whereby separate portions of the nitrogen to be repassed may be withdrawn through these lines simultaneously for transmittal as a combined stream to reversing valve 31. Valves are provided in each of lines 29, 291 and 292 to permit control of the amounts withdrawn through these three lines in accordance with the requirements of the process. A similar arrangement is provided in connection with lines 30, 301 and 392.

To determine the quantities of nitrogen to be withdrawn from the selected points provided by lines 29, 291 and 292, three separate calculations of the type described above may be followed. In the first calculation the connection of line 291 with the exchanger is considered to be the cold end of the exchanger and the calculation based on that assumption determines the quantity of nitrogen to be withdrawn through line 29. A second calculation is based on the assumption that the point of connection of line 292 with the exchanger is the cold end. This calculation determines the total uiil necessary to be repassed in conduit 14 between the connections of lines 292 and 291 with the exchanger. The difference between the amount thus determined and the amount withdrawn through line 29 is the amount to be withdrawn through line 291. A final calculation, similar to the one first described above, determines the maximum allowable temperature differential at the cold end of exchanger 12 and indicates the amount of nitrogen needed to be repassed in the exchanger between the cold end and the connection of line 292 with the exchanger. Subtracting from this amount the amounts withdrawn through lines 29 and 291 determines the amount necessary to be withdrawn through line 292. Necessarily the result of these calculations applies also to lines 30, 361 and 302.

In the foregoing discussion of the control of temperature differentials in the colder part of heat exchanger 12, the maximum temperature differentials consistent with complete evaporation of the carbon dioxide deposited from the air have been the basis for discussion and calculation. Ordinarily, in that part of the heat exchanger in which the carbon dioxide is precipitated, the maximum temperature differentials theoretically necessary to remove water vapor deposited as ice in that zone are somewhat lower than the maximum temperature differentials which would permit complete removal of carbon dioxide. Therefore, if it is desired to operate under conditions which would insure complete removal in each cycle of all the water, as well as all the carbon dioxide deposited in the zone of precipitation of carbon dioxide, it is necessary to base the calculations described above on water, rather than on carbon dioxide. It is possible to operate the heat exchanger so as to provide for full evaporation of water as well as carbon dioxide, but such operation results in a substantial increase in the size of the heat exchanger. In many instances it will be economical to operate with temperature differences such that carbon dioxide is evaporated completely and the minute quantities of water ice are evaporated incompletely in the zone of precipitation of carbon dioxide, the residual or unevaporated water being removed from the apparatus by thawing during the normal operational shutdowns.

Figure 2 illustrates a modification of the apparatus represented in Figure l. A portion of the apparatus of Figure l is reproduced in Figure 2 and those parts of Figure 2 which are substantially identical in construction and operation to similar parts in Figure l are indicated by the same reference numeral, with the subscript a. The detailed description of the construction and operation of such similar parts, included in the description of Figure l, applies also to corresponding parts of Figure 2. In the modification of Figure 2 the nitrogen to be repassed through the regenerating passageways of exchanger 12a is withdrawn from the nitrogen stream only after it has passed completely through the heat exchanger. The portion of the nitrogen stream thus diverted for repassage is treated to remove carbon dioxide and water vapor in a manner different than that illustrated in Figure l. In the operation of heat exchanger 12a, as in the operation of heat exchanger 12, the nitrogen stream passes through either of passageways 13a or 14a and either of lines 16a or 17a to valve 19a from which it is withdrawn from the system through line 20a. A portion of the nitrogen flowing from heat exchanger 12a through line 20a is diverted for repassage through the heat exchanger 12a, in accordance with the improved process of this invention. For this purpose line S1, provided with Valve S2, connects line 20a with drum 33a. The diverted nitrogen flows through line 81 into drum 33a and then through line S3 to blower 35a. Blower 35a discharges the diverted nitrogen through line 84 into a treating tower 85 in lwhich the gas ows upwardly in countercurrent contact with descending treating solution for removal of carbon dioxide. The treating solution may consist, for example,

aveavol of a 19% solution of potassium hydroxide or sodium hydroxide. Line 86 is provided for introducing such treating solution into the top tower 85, which is provided with vapor-liquid contact means, such as bubble cap trays 87. The treating solution flows downwardly into tower S in contact with the upflowing gas to absorb and remove therefrom the carbon dioxide impurity. The treating solution, containing absorbed carbon dioxide, is withdrawn from the bottom of the tower 85 through line 88.

The treated gas, substantially free of carbon dioxide, is withdrawn overhead from tower 85 through line 89. The treated gas is next subjected to drying to remove water vapor. For this purpose a pair of driers 90 and 91 are provided to afford contact of the gas with granular adsorbent material, such as silica gel or active alumina. Line 89 connects with line 92, which is provided with valve 93 through which the gas may be passed into drier 90. Front drier 90 the dry gas passes through line 94, provided with valve 95, into line 96, which connects in turn with line 28a. Line 89 also connects with line 97, provided with valve 9S, through which the gas from tower 85 is passed alternately into drier 91. Freni drier Sti the dry gas passes through line 99, provided with valve loo, into li'ne 96. Drie'rs 90 and 91 are employed in alternate drying and regeneration periods. Regeneration of the driers may be accomplished by withdrawing a portion of cool compressed air from line 26a through line 48a, warming this portion of air, and then conducting the warm air through the drier to be regenerated. Valves to lines 101, 102, 103 and 104 are provided to permit passage of the regenerating air through the drier to be regenerated. The puried and dried nitrogen stream owing through line 96 is commingled with the cold nitrogen from the fractionating system through line 28a.

The modication of Figure 2 differs from that of Figure l in the method for handling the carbon dioxide and water content of the nitrogen stream to be repassed. The conditions governing the quantity of nitrogen togbe diverted and repassed in accordance with the modication of Figure 2, are the same as those which govern the quantity of nitrogen to be repassed in accordance with modiiication of Figure 1, so that detailed discussion of this point in connection with Figure 2 is unnecessary. The handling of the diverted nitrogen in tower 85 and driers 90 and 91 connecting lines associated therewith should be carried out under conditions whichavoid, as far as possible, any heating of the gas. The substantial removal of CO2 and water vapor from the nitrogen stream to be repasse'd, in the modication of Figure 2, permits 'some control of the temperature of the repassed nitrogen, which is less feasible in the modication of Figure 1. Although the nitrogen may be returned directly to line 28a, it may be desirable to cool the nitrogen stream flowing through line 96 by heat exchange in heat exchanger 96x with any suitable cooling fluid, such as the air flowing through line 26a or the oxygen owing through line 75a. By this means the warming of the nitrogen stream in line 28a', which results from admixture of nitrogen from line 96 therewith, is substantially minimized or avoided.

The operations illustrated by Figures l and 2 involve exclusively the diversion and repassage of a portion of the counterflowing stream of cold nitrogen to control the temperature in a portion of the reversing heat exchanger in which there might otherwise exist excessive diiferentials between the temperatures of the warm streams a'nd the temperatures of the cold streams. Fig'- ure 3 illustrates, in part, a modication of the invention in which such temperature control is achieved by diverting and repassing a portion of a cold stream tiowing i'n non-reversing heat exchange with the air stream. As are the rnodications of Figures l and 2, the modification of Figure 3 is based on the operation of a reversing heat exchanger in which separate streams of nitrogen and oxygen products are passed in heat exchange with the compressed air, `the air stream and the nitrogen stream being passed alternately through reversing pa'ssageways to permit scavenging the air passageways of deposited water and carbon dioxide. As the heat exchanger and connecting lines associated therewith are substantially similar in construction and operation to those in Figures 1 and 2, the parts of Figure 3 which are thus similar in construction and operation to parts of Figure l are identified by the same reference numerals with the subscript b. The detailed description of the construction and operation of such similar parts, included in the description of Figure l, applies also to corresponding parts of Figure 3. Thus heat exchanger 12b is provided with reversing passageways 13b and 14h for the air and nitrogen streams and a non-reversing passageway 15b for the oxygen stream. In heatexchang'er 12b the position of passageways 13 and 15 is the reverse of the arrangement of the corresponding passageways of Figures l and 2.

In Figure 3 the oxygen-rich product stream flowing through line b passes into the entrance of passageway '15b at the cold end of heat exchanger 12b. This cooling stream ows through the heat exchanger countercu'rrent Atherrnal contact with the air stream and lis withdrawn from the warm end of the exchanger, and passageway 15b, through line 761:, in the manner described in connection with Figure 1. In accordance with this invention the temperature diier'entials between the cold and warm streams in the colder portion of heat exchanger 12b are controlled by withdrawing and repassing through the heat exchanger a. portion of the oxygen stream owing in heat exchange with the air. The portion of the oxygen stream to be diverted may be withdrawn at an intermediate point of passageway 15b. For this purpose line 10S, provided with valve 106, is providexl to connect the interior of passageway 15b with the inlet of a blower 15S. From the exit of blower 158 the diverted oxygen is passed through line 109 for repassage through that portion of heat exchanger 12b whose temperature `differences are to be controlled. ln the simplest embodiment of this type of operation the diverted oxygen would be made to pass from line 109 directly into line 75l), for repassage through the heat exchanger. For this purpose line 15S, provided with a valve 156, is arranged to connect line 109 with line 75b. Preferably, however, in the present illustrative example because of the character of the diversion, the diverted oxygen stream owing through line 109 is Vcooled prior to repassage and for this purpose line 109 is connected to the inlet of the heat exchanger 148. Alternatively the flow of the diverted oxygen stream is divided between line and heat exchanger Y148, the distribution of the oxygen in this manner being controlled by adjustment of valve 156 in line 155 and valve 147 in line 109. In heat exchanger 148 the oxygen introduced from line 109 ows in countercurrent heat exchange with at least a portion of the cold air from heat exchanger 12b. The cooled diverted oxygen emerges from heat exchanger 148 into line 149 through which it is conducted to line 75h. The cold compressed air is withdrawn from either of passageways 13b or 14h through lines 2'5b or 2711 both of which connect with line 26h, in the manner described in connection with Figure l. In the fractionation of air it is preferred to cool the air in the heat exchanger to a temperature which is lower than that desirable for the air to be sent to expander 70 of Figure l, and warm the portion of cold air sent to expander 70 suiciently to prevent condensation in the expander which might otherwise occur. vIn the modication of Figure 3 at least a portion of the diverted oxygen stream is utilized to warm at least a portion of the air to be sent to the expander. Air to be expanded is withdrawn from line 26h through line 150, which is provided with valve 151 and connects with a passageway of heat exchanger 148. The cold air thus introduced -in heat exchanger 143 passes in countercurrent heat exchange with the diverted oxygen stream flowing therethrough and is warmed in accordance with requirements of the process` sageway b.

The warmed air stream emerges from heat exchanger 148 through line 154 for passage to expansion with performance of external work, as illustrated by the operation of expander 70 of Figure 1. rl`he remainder of the compressed air passes from line 2611 into line 152, provided with a valve 153, for passage to liquefaction treatment illustrated by the operation of liquefier 56 in Figure 1. If desired line 160 may be provided to connect line 152 with line 154 whereby colder air may be passed from line 152 directly into line 154, or vice versa, to further adjust the temperatures of the air streams flowing through these lines. For example, the operation of heat exchanger 148 may require an exit temperature for the air stream which is too high for passage to the expander. Therefore, only a part of the air to be passed to the expander is warmed in heat exchanger 14S and the total quantity of air to be expanded is obtained, in the proportions adapted to produce the desired temperature, from both heat exchanger 148 and line 160.

Instead of, or in addition to, the heat exchange of the diverted oxygen with air in exchanger 148, at least a portion of the diverted oxygen may be passed in heat exchange with cold nitrogen in heat exchanger 145. For this purpose line 143, provided with valve 144, connects line 109 with the entrance of a passageway of heat exchanger 145. The oxygen thus introduced in heat exchanger 145 ows therethrough in heat exchange with cold nitrogen from the fractionation system and then passes through line 146 into line 75b. In this arrangement at least a portion of the nitrogen stream fiowing through line 28h is diverted through line 161, in amount controlled by valve 163, and passed into heat exchanger 145 through which it ows in countercurrent heat exchange with at least a portion of the diverted oxygen. The nitrogen stream emerges from heat exchanger 145 through line 162, through which it is returned to line 28h at a point nearer the entrance of heat exchanger 12b. Line ZSb connects with lines 25b and 27b in the manner described in connection with Figure l in order to direct the nitrogen stream into either of passageways 13b or 14b. In an alternative arrangement the diverted oxygen may be passed successively in heat exchange with both the cold air and the cold nitrogen. For example, the oxygen may be passed in series through heat exchangers 148 and 145.

Instead of withdrawing the oxygen stream to be diverted from an intermediate point of passageway 15b, such stream may `be obtained from the oxygen stream after it has emerged from passageway 15b. For this purpose line 168, provided with valve 107, is arranged to connect line 76b with line 105.

The considerations which govern the location of the point of diversion of the oxygen stream, and the quantity to be thus repassed, are the same as those which apply in determining the location of the point of diversion of the nitrogen, and the quantity to be diverted, in the process of Figure l. Necessarily, at least a part of the oxygen to be repassed must be withdrawn from the oxygen stream after the stream has fiowed through the area of the heat exchange zone in which the temperature differentials would be excessive, but for the withdrawal and repassage of the oxygen. In the cooling of air this requires ordinarily that at least a part of the diverted oxygen shall be withdrawn from a point between the area of precipitation of carbon dioxide and the warm end or" the heat exchanger. In Figure 3 such a point is indicated diagrammatically by the connection of line 195 with pas- In determining the quantity of oxygen to be withdrawn through line 105 for repassage, or determining the quantity to be withdrawn from any selected point, such as through line 15.18, the method of calculation described in connection with the withdrawal and repassage of nitrogen in Figure 1 may be employed. The heat which must be absorbed by the repassed nitrogen or oxygen can be determined in accordance with the method described in connection with Figure 1. The quantity of oxygen which must be withdrawn and repassed at any selected point is the quantity which absorbs heat, to the extent necessary to apply the required correction to the operation, in being warmed from the temperature at which it is introduced into passageway 15b from line 75b to the temperature at which it is withdrawn at the point selected for diversion.

As in connection with Figure 1, it is desirable for operations of economy, to withdraw the oxygen required for repassage at the coldest possible point and it is further desirable to withdraw the oxygen from a plurality of points throughout the Zone of precipitation of carbon dioxide, that is, throughout that portion of the oxygen passageway which is in heat exchange with a zone of precipitation of carbon dioxide. This last-mentioned operation is analogous to the operation described in connection with lines 291 and 292 of Figure 1.

In general the operation of the modification of the process in which oxygen is withdrawn and repassed for temperature control, in accordance with this invention, is similar to and based on the same considerations as, the modification in which nitrogen is withdrawn and repassed. One difference arises from the fact that the oxygen diverted and repassed does not contain absorbed vapors of water and carbon dioxide, wherefore no means for handling these materials are necessary in connection with the modification thus far described in connection with Figure 3. The point of withdrawal'of the oxygen stream may be selected without regard to any possible precipitation of impurities when the diverted oxygen stream is mixed with colder oxygen, or otherwise lowered in temperature. Another difference resides in the fact that the proportion of the oxygen stream which normally is required to be withdrawn and repassed is somewhat larger than the corresponding proportion of the nitrogen stream in the modification of Figure 1. Consequently the warming effect of the diverted oxygen stream on the main oxygen stream upon recombination is substantially greater than the corresponding effect employing nitrogen for repassage. For this reason the cooling of the diverted oxygen stream provided at and 148 in Figure 3 is particularly desirable, in order that the combined oxygen stream may not be excessively higher than the temperature of the nitrogen stream for efficient operation of heat exchanger 12b. It will be understood that any suitable means for cooling the stream of diverted oxygen instead of, or in addition to, those provided at 145 and 148 may be employed. In general the diverted oxygen may be cooled by heat exchange with any colder fiuid flowing in any part of the fractionation system, which may be considered to comprise fractionating tower 60, of Figure l, and the lines and pieces of subsidiary equipment through which the oxygen, air and nitrogen streams flow in passing between the fractionating tower and the reversing heat interchange zone.

In a further alternative operation the correction of the temperature differentials in the heat interchange zone may be effected by simultaneously diverting and repassing portions of each of a plurality of cold streams flowing in heat exchange with an air stream. This modification is illustrated in Figure 3 in which there is provided, in addition to the means for diverting and repassing a portion of the oxygen stream, means for diverting and repassing at the same time a portion of the nitrogen stream. For this purpose lines 29b and 30b are provided to conneet reversing valve 311) with the interiors of passageways 13b and Mb in the manner described in connection with Figure 1. The portion of the counterfiowing nitrogen stream which is diverted in this manner passes from valve 31b through line 32b to blower 35b. From blower 35b the diverted nitrogen is discharged directly into line 2317, through line Sb. The selection of the point of withdrawal of the nitrogen stream and the quantity to be repassed and the method for such withdrawal and re- 'generator being scavenged by the oxygen stream.

23 passage of a portion of the nitrogen stream in Figure 3 are all the same as in Figure 1. To simplify the drawing, drum 33 and filters 38 and 39 of Figure 1 have no counterpart of Figure 3. It will be understood, however, that these may be included in the operation of Figure 3.

In the combined operation illustrated in Figure 3 the amounts and locations vof withdrawal of the nitrogen and oxygen streams are determined Vby the methods described above in connection with Figure l. The amount of kheat which should be absorbed by the uid to be repassed in the combined operation in order to apply the needed correction to the temperature diierentials in the heat exchanger is determined in accordance with the method described in connection with Figure l. The amount of heat to be absorbed by the repassed nitrogen and oxygen is distributed to these diverted streams in any desirable portion and the quantities of the oxygen and nitrogen streams necessary to be repassed to absorb the total needed heat to be absorbed in the desired proportion are then withdrawn and repassed in accordance with the described method of operation in Figure 3. In a preferred embodiment of this entire operation the nitrogen and oxygen streams diverted from heat exchanger 12b are both withdrawn at a plurality of points in accordance with the method described in connection with lines 291 and 292 of Figure l.

The combined operation illustrated in Figure 3 provides several advantages. One advantages resides in the reduction of the proportion of the oxygen stream needed to be repassed, wherefore the need for heat exchange of the diverted oxygen with colder uides prior to recombination thereof with the main stream of oxygen is reduced or voided. Another advantage of the combined operation, over an operation following only the repassage of nitrogen, arises from the fact that the reduced quantity of nitrogen needed to be repassed simplies or eliminates the solution to the problem presented by the water vapor and carbon dioxide content of the repassed nitrogen stream. Thus, in the combined operation it is 'possible to avoid the need for means equivalent to lters 38 and 39 of Figure l or treating tower 85 and driers 90 `and 91 of Figure 2. The combined operation also presents a possibility of maintaining the temperatures of the cold nitrogen and cold oxygen streams entering heat exchanger 12b at approximately the same level without resort to heat exchange.

In the foregoing detailed description of the invention 'by `reference to Figures l, 2 and 3 the diversion or repassage of a portion of a stream of cooling iluid owing through a reversing heat exchange zone has been illustrated by referring specifically to a reversing heat exchanger. As has been pointed out, the invention is applicable also to reversing heat interchange operations in which reversing regenerators are employed. This modi- `cation of the invention is illustrated in Figure 4.

In the arrangement illustrated in Figure 4 a pair of regenerators 110 and 111 is provided and the air and nitrogen streams are passed through these regenerators in reversing heat interchange. When compressed air to be `fractionated is cooled by heat interchange with cold products such heat interchange may be effected in two or more pairs of regenerators, the nitrogen and oxygen streams being passed through separate pairs of regenerators and the air feed stream being distributed among the .the various pairs of regenerators in the proper proportions. However, it may be desired to avoid using the oxygen stream for scavenging purposes, in order to avoid contaminating it with water vapor, carbon dioxide and the nitrogen component of the air remaining in the re- In order to utilize the cooling effect of the oxygen product stream in such circumstances, it is desirable to incorporate in the regenerators to be scavenged by the nitrogen stream 'separate passageways through which the oxygen product flows in non-reversing heat Yinterchange with the air. The latter arrangement is that illustrated in Figure 4.

In Figure 4 certain parts and lines are similar in construction and function to corresponding parts and lines of Figure l and Figure 2. Such corresponding parts are designated in Figure 4 by the same reference numerals as their counterparts in Figures l and 2, with the subscript c. As the construction and operation of those parts of Figure 4 identified by reference numerals having subscript c have been described in connection with Figures l and 2, no further detailed description on these parts of Figure 4 is necessary.

Figure 4 the compressed air from line 11C is passed into regenerator through line 112, reversing valve 113, and line 114, or into regenerator 111 through reversing valve 115 and line 116. The nitrogen product fraction passes from regenerator 111 through line 116, reversing valve 115, line 118, and line 120, or the nitrogen stream is withdrawn from regenerator 110 through line 114, reversing valve 113, and line 121). The cold compressed air emerges from regenerator 110 through line 121, which connects With lines 123 and 128. A check valve 122 is provided in line 123 and a check valve 129 is provided in line 128. Check valve 129 is arranged to be held closed by compressed air from line 121 while check valve 122 is held open by the compressed air, whereby the cold compressed air flows from line 121 through line 123 into line 26C. When the compressed air is owing through regenerator 111 it lemerges through line 124 which connects with lines 117 and 126. Check valve 125 is provided in line 126 and check valve 127 is provided in line 117 Check valve 125 is arranged to be held open by the compressed air from line 124 while check valve 127 is held closed by the compressed air, whereby the cold compressed air from line 124 liows through check valve 125 and line 126 into line 26C. The cold 'nitrogen stream from the fractionating system is supplied through line 28e which connects with lines 128 and 117. Valves 113 and 115 are operated periodically, by an automatic timing device not shown, whereby valve 113 directs compressed air from line 11e into regenerator 110 at the same time that valve 115 is operated to exclude compressed air from line 116 and permit ilow of the nitrogen product stream into line 118, and vice versa. Compressed air flowing out of regenerator 110vthrough line 121 holds valve 122 open while valves 129 and 125 are held closed by the air pressure. Consequently, the nitrogen owing through line 28e flows through valve 127 only, whereby the operation of valves 113 and 115 automatically directs the nitrogen stream from line 28C into that regenerator not receiving the compressed air from 11C.

The stream of cold oxygen from the fractionating systern is supplied through line 75a` which connects with lines 1311 and 131. These in turn connect, at the cold ends of the regenerators, with conduits 132 and 133, l0- cated in regenerators 110 and 111 respectively. Outlet lines 134 and 135 are connected to conduits 132 and 133, respectively at the warm ends of the regenerators. Lines 134 and 135 connect with reversing valve 136 through which the warmed oxygen stream is directed to line 137 for withdrawal from the system. Reversing Valve 136 may be operated cooperatively with valves 113 and 115 whereby the oxygen stream is passed through the conduit in one of the regenerators simultaneously with the ilow of air therethrough. ln this arrangement the air is cooled by heat exchange with the oxygen stream at the same time it is cooled regeneratively by the nitrogen. Alternatively, the oxygen stream may be passed through one of the regenerators simultaneously with the flow of nitrogen therethrough, whereby the air which subsequently passes through that regenerator is cooled regeneratively by both oxygen and nitrogen. As a further alternative, the apparatus of Figure 4 may be arranged to ow the oxygen continuously through both regenerators..

As in connection with the reversing heat exchangers described above, the product of the mass ow rate of the air and its specific heat is greater than the sum of the corresponding products for the oxygen and nitrogen cooling streams. Consequently, there is a tendency for the differentials between the temperatures of deposition and evaportion of water and carbon dioxide to become excessive in the colder portions of the regenerators, whereby the nitrogen stream does not evaporate in each period all the impurities, and particularly does not evaporate all carbon dioxide, laid down in the previous period. To correct this condition, in accordance with the improvement of this invention, a portion of the nitrogen stream owing through line 120 may be diverted therefrom through line 81e which is provided with a valve 82e and connects with surge drum 33e. From surge drum 33e the diverted nitrogen passes through line 83e to blower 35C from which the nitrogen stream is introduced by means of line 84e into the lower part of a caustic treating tower 85C. through line 89C, through either of driers 90e or 91C, and line 96e into line 28C. The operation of the means for treating and drying the diverted nitrogen which have been described thus far in connection with Figure 4 have been described in connection with Figure 2 so that no further description is necessary.

It will be apparent that the method for adjusting the temperature differentials provided by that part of Figure 4 thus far described is an adaptation of the method of Figure 2 to reversing regenerators. The considerations which govern the quantity of nitrogen necessary to be repassed in the operation of Figure 4 are similar to those described above in connection with the reversing heat exchanger.

While Figure 4 as thus far described provides for withdrawing and passing a portion of the nitrogen stream after it has passed completely through the regenerator it is clear that the operation of the reversing regenerators, in accordance with this invention, it not limited to this particular method. Alternatively, nitrogen can be diverted for repassage from one or more intermediate points of the regenerators. To permit this operation lines 139 and 140 are provided to provide access to the interiors of the nitrogen passageways of regenerators 111 and 110, respectively. Lines 139 and 140 connect with reversing valve i38 which, in turn, connects with line 141 through which nitrogen withdrawn from regenerators 110 and 111 through lines 140 and 139 is delivered to line 81C and the inlet to surge drum 33e. In this modification of the operation of Figure 4 the treatment of the diverted gas at 85C and 9) and 91C is dispensed with and the diverted nitrogen stream is discharged directly into line 28C from the exit blower 35C. For this purpose line 119 is provided to connect line 84C with line 28C. In this arrangement it may, under some conditions, be desirable to provide means for separating precipitated impurities in line 28e, analogous to the method provided in Figure 1 by filters 38 and 39.

While the apparatus of Figure 4 is arranged to provide only for the withdrawal and repassage of nitrogen to maintain proper temperature differentials in the regenerators, it is evident that such control may be provided alternatively or in addition by the withdrawal and repassage of a portion of the counterflowing oxygen stream in conduits 132 or 133 in accordance with the principles described in connection with a similar operation in Figure 3.

The considerations which govern the location and extent of diversion of the cooling fluid in the embodiment of Figure 4 are the same as those described above in connection with the reversing heat exchangers. In the regenerators the temperature at any one point varies continuously during each operating period whereby calculations must be based on the average temperature at that point during the period. The general rule is that the correction provided must be sufficient to maintain the dierence From the top of tower 85C the treated gas passes 26 between the average temperature at a selected point in the regenerator during the air ow and the average temperature during subsequent nitrogen flow past that point such that the capacity of the nitrogen stream for carrying in vapor from the impurity under consideration is substantially equal to or greater than that of the air stream.

In all specific examples described above the oxygen stream is passed in non-reversing heat exchange with the air stream. It will be understood, of course, that the oxygen stream may be passed in reversing heat exchange with at least a portion of the air stream, even in operations in which a portion of the oxygen stream is diverted and repassed through at least a part of the heat interchange zone in accordance with this invention. In such operations, it may be desirable to treat the oxygen product stream to remove carbon dioxide and Water vapor, for example, in the manner described in connection with similar treatment of the repassed nitrogen in Figure 2. For example, all the oxygen product from a heat interchange zone in which the oxygen is passed in reversing heat interchange with the air may be treated to remove carbon dioxide and water and a portion of the oxygen thus treated may be diverted for repassage.

This invention is applicable to all operations in which the available cooling uid is of a quantity and a condition Such that the product of the mass How rate and specific heat of the warm fluid is greater than the sum of the products of mass flow rate and specific heat of each cooling fluid. This condition generally applies in cases where the Warm fluid is cooled only by components thereof at lower pressure. This invention is applicable in all cases where, under such conditions, it is desired to control the differentials between the temperatures of the cold and warm streams near the cold end of the heat interchange zone.

In the foregoing examples, relating to air fractionation, the compressed air is cooled by heat exchange with substantially all the products of fractionation. In such cases the Withdrawal and repassage of a portion of counterflowing uid necessarily has the result that in that portion of the interchange zone traversed by the diverted portion of cooling fluid the mass ilovv rate of the cold streams necessarily is greater than the mass flow rate of the warm stream. In operations in which a product is withdrawn in liquid form, such as operations involving the production of liquid oxygen or liquid air, the mass flow rate of the cold product stream, or streams, necessarily is less than the mass flow rate of the warm stream. Nevertheless it is found that in these operations suicient diversion and repassage of cooling fluid to effect the necessary temperature control results in a condition, in that portion of the heat interchange Zone traversed by the diverted portion of cooling fluid, in which the sum of the mass ow rates of the cooling fluids is greater than the mass llow rate of the warm uid.

The invention has been described in detail by reference to the treatment of air. It will be understood, however, that the invention is broadly applicable to the treatment of any gas of mixture of gases which treatment involves problems analogous to those solved by the application of this invention to the treatment of air. For example, the invention is applicable in the fractionation treatment of other gas mixtures such as mixtures comprising light hydrocarbons, hydrogen, nitrogen and helium. The invention also is applicable to operations involving the treatment of air other than the liquefaction and/or fractionation thereof, such as operations for preparing specimens oi' air having general characteristics of the earths atmosphere at different altitudes.

We claim:

1. In a process for fractionating a compressed gaseous mixture in a low-temperature expansion and fractionating system, wherein an inilowing charge stream of said compressed gaseous mixture enters said system at a pre-expansion pressure through a reversing heat exchange zone in Which said iniiowing stream is cooled and in a cold part of which high-boiling impurities are precipitated, and 'wherein an outflowing product stream leaves said system at a post-expansion pressure through said reversing heat exchange zone to absorb heat and scavenge said precipitated high-boiling impurity by revaporization, said inowing and outowing streams being 'o'wed conntercurrently and alternately with each other through periodically reversing paths in a heat exchange relation in said reversing heat exchange zone, a method for preventing the excessive accumulation of said precipitated impurity in said cold part of the paths of said reversing heat exchange zone during several cycles of operation, which method includes the steps of: withdrawing a part of said out iowing 'scavenging product subsequent to its passage through at least part of said region of precipitation of high-boiling impurity; and returning said withdrawn part to said outtl'owing scavenging stream at a point upstream from said point of withdrawal.

2. ln a process for separating air into at least oxygenrich and nitrogen-rich product fractions in a low-temperature expansion and fractionating system, wherein an inowing charge stream of air enters said system at a preexpansion pressure through a reversing heat exchange zone in which said inowing stream is cooled and in a cold part of which carbon dioxide is precipitated, and wherein an outllowing nitrogen-rich stream leaves said system at a post-expansion pressure through said reversing heat exchange zone to absorb heat and scavenge said precipitated carbon dioxide by revaporization, said iniiowing and outflowing streams being flowed countercurrently and alternately with each other through periodically reversing paths in a heat exchange relation in said reversing heat exchange Zone, a method for preventing the excessive accumulation of said precipitated carbon dioxide in said cold part of the paths of said reversing heat exchange zone during several cycles of operation, which method includes the steps of: withdrawing from said reversing heat exchange zone a sidestream comprised of part of said outilowing nitrogen-rich product subsequent to its passage through at least part of said region of precipitation of carbon dioxide; and returning said Withdrawn sidestream to said outowing scavenging stream at a point upstream from said region of precipitation and scavenging.

3. In a process for fractionating a compressed gaseous mixture in a low-temperature expansion and fractionating system, wherein an inowing charge stream of said compressed gaseous mixture enters said system at a preexpansion pressure through a reversing heat exchange zone in which said inowing stream is cooled and in a cold part of which high-boiling impurities are precipitated, and wherein an outiiowing product stream leaves said system at a post-expansion pressure through said reversing heat exchange zone to absorb heat and scavenge said precipitated high-boiling impurity by revaporization, said inowing and outilowing streams being owed countercurrently and alternately with each other through periodically reversing paths in a heat exchange relation in said reversing heat exchange zone, a method for preventing the excessive accumulation of said precipitated impurity in said cold part of the paths of said reversing heat exchange zone during several cycles of operation, which method includes the steps of: withdrawing from said reversing heat exchange zone at an intermediate point and downstream from the region of said excessive accumulation a sidestream comprised of a part of said outflowing scavenging stream; compressing said .withdrawn sidestream to the pressure of said outilowing scavenging stream at a point upstream from said region of excessive accumulation; and returning said compressed sidestream to said outflowing stream at said last mentioned point,

4. In a process for fractionating a compressed gaseous mixture in a low-temperature expansion and fractionatiug system, wherein an infiowing charge stream of said u compressed gaseous mixture enters said system at a preexpansion pressure from a reversing heat exchange one in which 'said inowing stream is cooled, and in a cold part of which high-boiling impurities are precipitated, and wherein at least one outtiowing product stream leaves said system at a post-expansion pressure through said reversing heat exchange zone, absorbing heat and scavenging said precipitated high-boiling impurities by revaporization, said inflowing and outflowing streams being flowed countercurrently and alternately with each other through periodically reversing paths in a heat exchange relation in said reversing heat exchange zone, a method for preventing the excessive accumulation of said highboiling impurity in said cold part, which includes the steps of: withdrawing a sidestream from said outflowing scavenging product stream at a point subsequent to its passage through at least part of said cold portion; compressing said withdrawn sidestream back to the pressure of said outflowing stream prior to its entrance into said reversing heat exchange Zone; treating said compressed stream to remove high-boiling impurity therefrom; and reintroducing said treated stream into said outilowing product scavenging stream at a point upstream from said region of 'precipitation and scavenging.

5. In a process for fractionating a compressed gaseous mixture in a low-temperature expansion and fractionating system, wherein an inilowing charge stream of said compressed gaseous mixture enters said system at a pre-expansion pressure through a reversing heat exchange zone in which said inowing stream is cooled and in a cold part of which high-boiling impurities are precipitated, and wherein an outowing product stream leaves said system at a post-expansion pressure through said reversing heat exchange zone to absorb heat and scavenge said precipitated high-boiling impurity by revaporization, said inflowing and outowing streams being owed countercurrently and alternately with each other through periodically reversing paths in a heat exchange relation in said reversing heat exchange Zone, a method for preventing the excessive accumulation of said precipitated impurity in said cold part of the paths of said reversing heat exchange zone during several cycles of operation, which method includes the steps of: withdrawing from said reversing heat exchange zone a sidestream comprised of a part of said outtlowing scavenging stream subsequent to its passage through said region of precipitation of high boiling impurity; compressing said withdrawn sidestream to the pressure of said outowing scavenging stream at a point upstream from said region of precipitation and scavenging; combining said compressed side-stream with said outflowing product scavenging stream at a point upstream from said reversing heat exchange zone; and filtering precipitated high-boiling impurities from said combined outowing product scavenging stream prior to passage thereof into said heat exchange zone.

6. in a process for separating air into at least oxygenrich and nitrogen-rich product fractions in a low-temperature expansion and fractionating system, wherein an inowing charge stream of air enters said system at a preexpansion pressure through a reversing heat exchange zone in which said inowing stream is cooled and in a cold part of which carbon dioxide is precipitated, and wherein an outowing nitrogen-rich stream leaves said system at a post-expansion pressure through said reversing heat exchange zone to absorb heat and scavenge said 'precipitated carbon dioxide by revaporization, said inowing and outowing streams being flowed countercurrently and alternately with each other through periodically rcversing paths in a heat exchange relation in said reversing heat exchange zone, a method for preventing the excessive accumulation of said precipitated carbon dioxide in said cold part of the paths of said reversing heat exchange Zone during several cycles of operation, which method includes the steps of: withdrawing from said reversing heat exchange Zone a sidestream of said outowing nitro- 29 gen-rich stream at a point having a temperature in the range of 185 F. up to -80 F.; and returning said sidestream to said outflowing scavenging stream at a point upstream from said point of withdrawal.

7. In a process for separating air into at least oxygenrich and nitrogen-rich product fractions in a low-temperature expansion and frictionating system, wherein an inowing charge stream of air enters said system at a preexpansion pressure through a reversing heat exchange zone in which said inowing stream is cooled and in a cold part of which carbon dioxide is precipitated, and wherein an outowing nitrogen-rich stream leaves said system at a post-expansion pressure through said reversing heat exchange zone to absorb heat and scavenge said precipitated carbon dioxide by revaporization, said inowing and outilowing streams being flowed countercurrently and alternately with each other through periodically reversing paths in a heat exchange relation in said reversing heat exchange Zone, a method for preventing the excessive accumulation of said precipitated carbon dioxide in said cold part of the paths of said reversing heat exchange zone during several cycles of operation, which method includes the steps of: withdrawing from said reversing heat exchange Zone a sidestream of said outflowing nitrogen-rich stream at a point downstream from said region of precipitation of carbon dioxide; contacting said sidestream with an aqueous caustic solution to remove carbon dioxide; passing said stream through driers to remove water vapor; and recombining said stream with said outowing scavenging stream at a point upstream from said cold part in which precipitation and scavenging occur.

8. In a process for fractionating a compressed gaseous mixture in a low-temperature expansion and fractionating system, wherein an inowing charge stream of said compressed gaseous mixture enters said system at a preexpansion pressure through a reversing heat exchange zone in which said inflowing stream is cooled and in a cold part of which high-boiling impurities are precipitated, and wherein an outowing product stream leaves said system at a post-expansion pressure through said reversing heat exchange zone to absorb heat and scavenge said precipitated high-boiling impurity by revaporization, said inowing and outflowing streams being owed countercurrently and alternately with each other through periodically reversing paths in a heat exchange relation in said reversing heat exchange Zone, a method for preventing the excessive accumulation of said precipitated impurity in said cold part of the paths of said reversing heat exchange zone during several cycles of operation which method includes the steps of: withdrawing a sidestream from said outflowing product scavenging stream at a point downstream from said cold part in which precipitation and scavenging occur; similarly withdrawing at least one additional sidestream from said cold part of said reversing heat exchange zone at a point upstream from said downstream sidestream; and returning said sidestreams to said outilowing product scavenging stream upstream from said cold part.

9. In a process for fractionating a compressed gaseous mixture in a low-temperature expansion and fractionating system, wherein an inlowing charge stream of said compressed gaseous mixture enters said system at a preexpansion pressure through reversing heat exchange Zone in which said inilowing stream is cooled and -in a cold part of which high-boiling impurities are precipitated, and wherein an outowing product stream leaves said system at a post-expansion pressure through said reversing heat exchange Zone to absorb heat and scavenge said precipitated high-boiling impurity by revaporization, said inflowing and outowing streams being ilowed countercurrently and alternately with each other through periodically reversing paths in a heat exchange relation in said reversing heat exchange zone, a method for preventing the excessive accumulation of said precipitated impurity in said cold part of the paths of said reversing heat exchange zone during several cycles of operation by restricting the temperature differences between the counterllowing streams, which method includes the steps of: withdrawing a sidestream of outflowing product scavenging stream from said reversing heat exchange zone at a point downstream from the region of said excessive accumulation of precipitated impurity; similarly withdrawing at least one additional sidestream from said cold part of said reversing heat exchange Zone upstream from said first mentioned sidestream; returning said sidestreams to said outowing product scavenging stream upstream from said region of excessive accumulation; maintaining the ow of each such additional sidestream from said cold part at a mass flow rate which reduces the mass iiow rate of product scavenging stream downstream from said sidestream sufficiently so that an increasing difference in temperature between said counterflowing streams is obtained as the outflowing product scavenging stream flows through a substantial portion of the heat-exchange zone upstream from said first mentioned sidestream, whereby the allowable temperature difference between said counterflowing streams is approached more closely than is possible with a single sidestream.

10. In a process for separating air into at least oxygenrich and nitrogen-rich product fractions by compressing, expanding, liquefying and evaporating at least part of said air, in a fractionating system, wherein an inflowing stream of compressed air is cooled by direct heat exchange with one of a pair of regenerators, precipitating carbon dioxide within said regenerator in a cold portion thereof, while outowing nitrogen-rich product is simultaneously countertlowed through the second of said regenerators, refrigerating said second regenerator and scavenging precipitated carbon dioxide therefrom, and wherein the flow in said regenerators is periodically interchanged so that each experiences alternate charging and refrigerating periods, a method for preventing the excessive accumulation of said precipitated carbon dioxide in said cold portions of said regenerators during several cycles of operation which method includes the steps of: continuously withdrawing as a sidestream part of said outflowing nitrogen-rich product stream after its outward passage through at least part of said cold portion during said refrigerating period; and returning said withdrawn part to said outowing scavenging stream at a point upstream trom said point of withdrawal.

References Cited inthe file of this patent UNITED STATES PATENTS 2,513,306 Garbo July 4, 1950 2,586,811 Garbo Feb. 26, 1952 2,650,481 Cooper Sept. 1, 1953 2,663,170 Gloyer Dec. 22, 1953 

1. IN A PROCESS FOR FRACTINATING A COMPRESSED GASEOUS MIXTURE IN A LOW-TEMPERATURE EXPANSION AND FRACTIONATING SYSTEM, WHEREIN AN INFLOWING CHARGE STREAM OF SAID COMPRESSED GASEOUS MIXTURE ENTERS SAID SYSTEM AT A PRE-EXPANSION PRESSURE THROUGH A REVERSING HEAT EXCHANGE ZONE IN WHICH SAID INFLOWING STREAM IS COOLED AND IN A COLD PART OF WHICH HIGH-BOILING IMPURITIES ARE PRECIPITATED, AND WHEREIN AN OUTFLOWING PRODUCT STREAM LEAVES SAID SYSTEM AT A POST-EXPANSION PRESSURE THROUGH SAID REVERSING HEAT EXCHANGE ZONE TO ABSORB HEAT AND SCAVENGE SAID PRECIPITATED HIGH-BOILING IMPURITY BY REVAPORIZATION, SAID INFLOWING AND OUTFLOWING STREAMS BEING FLOWED COUNTERCURRENTLY AND ALTERNATELY WITH EACH OTHER THROUGH PERIODICALLY REVERSING PATHS IN A HEAT EXCHANGE RELATION IN SAID REVERSING HEAT EXCHANGE ZONE, A METHOD FOR PREVENTING THE EXCESSIVE ACCUMULATION OF SAID PRECIPITATED IMPURITY IN SAID COLD PART OF THE PATHS OF SAID REVERSING HEAT EXCHANGE ZONE DURING SEVERAL CYCLES OF OPERATION, WHICH METHOD INCLUDES THE STEPS OF: WITHDRAWING A PART OF SAID OUTFLOWING SCAVENGING PRODUCT SUBSEQUENT TO ITS PASSAGE THROUGH AT LEAST PART OF SAID REGION OF PRECIPITATION OF HIGH-BOILING IMPURITY; AND RETURING SAID WITHDRAWN PART TO SAID OUTFLOWING SCAVENGING STREAM AT A POINT UPSTREAM FROM SAID POINT OF WITHDRAWAL. 